That first effort was initially panned on social media, with users citing low inventories and deals on obscure products that only appealed to scavengers. This time, shoppers vented their frustration on social media using the hashtag, #PrimeDayFail, with many saying they couldn't add items to their digital baskets.

"@Amazon my cart continues to fail. Very disappointed. There are some great deals that I'm missing. #PrimeDay #PrimeDayFail #AmazonPrimeDay," wrote Giselle Mazur in a Twitter post.

Even with last year's backlash, analysts said the event helped Prime membership and built the foundation for a strong holiday shopping season in 2015. James Cakmak, an analyst at Monness, Crespi, Hardt & Co, estimated, based on last year's success, that technical difficulties wouldn't hamper sales for too long this time.

"It will serve as a tail wind for the third quarter," Mr Cakmak said in an interview. "You have a perfect storm heading into the day, and while technical difficulties may get in the way in the short term, it shouldn't affect revenue."

Mike Pachter, an analyst at Wedbush Securities Inc in Los Angeles, said the problems indicate Amazon is seeing tremendous demand from shoppers.

"The biggest cloud manager on the planet having issues? That's a testament to high demand and high expectations for reliability," Mr Pachter said in an e-mail.

The company's shares fell 0.3 per cent to US$751.91 at 11:16 am in New York. They had climbed 12 per cent this year through Monday.